The present invention relates generally to the field of laser interferometry and more particularly to an improved system for accurately positioning a mechanical arm using a laser interferometer. The predominant current usage of the non-contact servo track writing apparatus is in the positioning of servo track writing heads in the manufacture of very small computer hard disk drives.
When manufacturing computer hard disk drives it is necessary to magnetically xe2x80x9cwritexe2x80x9d servo tracks on the disks, which servo tracks constitute the references from which other positions on the disks are located. Given the very high track density of modem hard disk media, it is critical that these servo tracks be accurately located and, therefore, it is necessary to position the servo track writing apparatus over the disk as precisely as possible during this process. Laser interferometry has been widely used in this endeavor due to the high resolution, high repeatability, great accuracy and fast signal updating rate of this technology. A typical prior art configuration has included a retro-reflector mounted on the positioning arm of the disk drive with the beam of the laser interferometer directed into the retro-reflector such that movement of the retro-reflector may be detected and measured by the interferometer. In this prior art method, the positioning arm has an aperture into which a pin may be placed for moving the arm to properly locate the head for writing the servo track. This method has worked perfectly well for most existing hard disk media. However, the demand for smaller and smaller hard disk media has now resulted in the desire to produce hard disk media smaller even than two inches in diameter. Currently, there exists media of 1.8 inches and smaller in various stages of production and pre production planning. With such very small media disks, not only is it increasingly critical that the servo tracks be accurately placed (since the actual physical track density is increased as the size of the media is decreased) the problem is also introduced that an arm for positioning the write head must be so small that it is difficult to attach a retro-reflector thereto. Furthermore, the entire hard disk mechanism is so small that it is not possible to introduce any bulky equipment into the area of the servo writing mechanism to position the arm.
It is known in the art to use a laser interferometer to position devices in a non-invasive manner. U.S. Pat. No. 5,108,184, issued to Brown et al., teaches a variation of the method of using a reflective surface of a device to gauge the position of that device with a laser interferometer.
Further, it is known to provide a non-invasive means for positioning a servo track writing device using a laser interferometer. For example, the present inventor""s U.S. Pat. No. 5,315,372 teaches one method of accomplishing this goal using a movable arm having an interferometer target, separate from the read/write head positioning arm of a workpiece disk drive. At the time of the inventor""s prior application no prior art device was readily adaptable to control very small positioning arms while providing the same degree of accuracy found in direct reading interferometers having the retro-reflector mounted on the head positioning arm. Even to date, all prior art non-invasive means require either a large reflective surface than is available in this application, or else have sacrificed accuracy by being highly dependent upon the flatness of the reflective surface, or else have not provided sufficient resolution for the writing of servo tracks on very small media.
The present inventor""s ""372 patent did much to advance the art of servotrack writing, but it did so at the expense of still using a flat reflective surface on the small positioning arm. Further, due to the desirability of sensing the positioning arm at a radially displaced point to maximize resolution, accessibility problems for interferometer beams became problematic. The inventor""s ""372 patent also discussed, briefly, the possibility of using a common positioning arm feature called a through hole as a sensing target, but initial embodiments utilizing that concept provided less resolution than the main ""372 technique. Thus practical read/write arm position sensing which was truly non-invasive and direct remained an industry goal. To the inventor""s knowledge, no prior art device has yet to provide both an efficient means to directly and accurately determining the position of a very small positioning arm while also providing a means for adjusting such position.
Accordingly, it is an object of the present invention to provide a means for positioning a servo track writing device which is adaptable for use with very small media disks and disk drive assemblies.
It is another object of the present invention to provide a means for positioning a servo track writing device which can very accurately position the writing heads.
It is still another object of the present invention to provide a means for positioning a servo track writing device which will direct the laser beam along a narrow path to and from the writing device.
It is yet another object of the present invention to provide a servo track writing device which will both accurately determine the location of a head positioning arm and also cause such arm to move to a correct position.
It is still another object of the present invention to provide a servo track writing device which is adaptable for use with various types of head positioning actuators.
It is yet another object of the present invention to provide a servo track writing device which is inexpensive to produce and reliable in operation.
It is still another object of the present invention to provide a servo track writing device which can be attached to a disk drive assembly, used, and detached quickly, so as to facilitate assembly line production of disk drive devices.
Briefly, the preferred embodiment of the present invention is a servo track writing apparatus for accurately positioning a magnetic head mounted on a positioning arm. The positioning arm is located within a disk drive assembly workpiece and has a through hole, an artifact of prior art positioning systems present in must positioning arms still in use today. The positioning arm is movable by a positioning means inherent to the disk drive workpiece to position the magnetic head over a rotating magnetic media. The inventive writing apparatus has a tracking arm located external to the disk drive assembly which has its own motive means, suitable for moving the tracking arm in a fixed relationship with the positioning arm of the disk drive assembly. The tracking arm has mounted on it a detection system for detecting the edges of the through hole in the positioning arm and thereby maintaining it in a fixed positional relationship with the positioning arm inside the disk drive assembly. A position determining system is provided and used to determine the position of the tracking arm, and to therefrom infer the position of the magnetic head within the disk drive assembly.
An advantage of the present invention is that it retains the accuracy of conventional laser interferometer head positioning devices.
A further advantage of the present invention is that it can be used with very small head positioning arms and very small magnetic media disks.
Yet another advantage of the present invention is that it is not necessary to mechanically connect any positioning apparatus to the head positioning arm.
Still another advantage of the present invention is that it is inexpensive to produce.
Yet another advantage of the present invention is that it can be easily adapted for use with different types of disk drives.
Still another advantage of the present invention is that it is readily adaptable to disk drive production environments.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known modes of carrying out the invention and the industrial applicability of the preferred embodiments as described herein and as illustrated in the several figures of the drawing.